We examined expression and production of interleukin-3 (IL-3) mRNA and IL-3 protein in mouse primary cultured neurons and glia by the reverse transcription and polymerase chain reaction and a bioassay using an IL-3-dependent cell line. IL-3 mRNA was demonstrated mainly in hippocampal neurons but not in glia, while a small but definite production of bioactive IL-3 was detected in septal and hippocampal neuronal cultures. Thus, endogenous IL-3 might be produced by certain neurons in situ.